I've recently and before visited my favorites list from different devices occasionally and noticed there are posts that I don't even remember seeing on e621.net are found in my favorites list. HOWEVER, everytime I remove them, I scroll down, they appear again, sometimes including blacklisted tags

even though I have strong password and don't share my account access to anyone else

edit: I'm on mobile

Use of the account is your responsibility ... I saw that somewhere and what a cold comment.

Having said that,
It's important to have a strong password,
but also important to change it once you feel it's weird.

It's a hassle, but you might want to review your favorites.
Maybe then you'll get to the bottom of it, and it's very bad if someone is really using your account illegally.